This invention relates to a direct positive light-sensitive silver halide photographic material, particularly to a direct positive light-sensitive silver halide photographic material excellent in stability with lapse of time.
The methods to be used for preparing positive images by use of the direct positive type light-sensitive material known in the art can be classified mainly into the two types.
One type is to use a silver halide emulsion previously fogged and obtain a positive image after developing by destroying the fogged nucleus (latent image) at the exposed portion by utilizing solarization or Herschel effect, etc.
The other type is to use the inner latent image type silver halide emulsion not previously fogged and obtain a positive image by effecting surface development after applying the fogging treatment subsequent to image exposure after applying the fogging treatment or while applying the fogging treatment.
Here, the fogging treatment may be performed by giving the whole surface exposure or chemically by use of a foggant, or alternatively by use of a strong developer or heat treatment, etc., but ordinarily the method by use of light or a foggant is employed. The inner latent image type silver halide photographic emulsion refers to a silver halide photographic emulsion, having light-sensitive nuclei primarily internally of the silver halide grains which forms latent images internally of the grains by exposure.
The method of the latter type is generally higher in sensitivity as compared with the former type and suitable for application in which high sensitivity is required, and particularly the present invention concerns the latter type.
In this field of the art, there have been heretofore known various techniques. For example, those as described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,761,266, 3,761,276, 3,796,577 and U.K. Patent No. 1,151,363 are the main ones.
By use of these known methods, light-sensitive materials of relatively higher sensitivity can be prepared as the direct positive type.
Also, as to the formation mechanism of positive image, it may be considered as follows, as described in Photographic Science and Engineering, vol. 20, p. 158, 1976. That is, the photoelectrons formed within the silver halide grains by image exposure are selectively captured internally of the grains to form an inner latent image. Since the inner latent image acts as effective capturing centers for the electrons existing in the electroconduction band, the electrons to be injected in the subsequent fogging development process will be captured internally to intensify the latent image. In this case, since the latent images are all within the grains and therefore will not be developed. On the other hand, in the grains not subjected to image exposure, at least a part of the electrons injected are captured on the grain surface, and said grains are developed through surface development.
By use of the above known techniques, although it is possible to prepare a light-sensitive photographic material forming a positive image, further improvement of the photographic performances is desirable for applying these light-sensitive photographic materials in various fields of photography.
For example, in recent years, by utilizing a copying device with a printed manuscript or a photography as the original, it has become possible to copy an image directly on the above direct positive type color paper or color film. For copying devices of such photographic system to be propagated widely in various uses, it is strongly desirable to shorten further the processing time of the light-sensitive photographic material.
In the prior art, for shortening the processing time of light-sensitive color photographic material, various methods have been employed. As is well known in the art, among silver halide crystals, silver chloride crystals are higher in ion crystallinity and also relatively higher in water-solubility than other silver bromide or silver iodide, and they have the characteristic of rapid developing, bleaching and fixing reactions. Accordingly, by use of silver halide with high silver chloride content, all the processings of developing, bleaching and fixing can be performed rapidly to be advantageous for shortening of the processing time.
The inner latent image type silver halide grains containing silver chloride are also well known, and one of the core/shell structure is disclosed in Japanese Unexamined Patent Publication No. 32820/1972 and also silver halide crystal grains of the structure having a layer with higher content of silver chloride laminated in Japanese Unexamined Patent Publications Nos. 8524/1975 and 38525/1975.
On the other hand, although the maximum density of the direct positive image obtained by use of the inner latent image type silver halide emulsion comprising primarily silver chloride is high, the minimum density (fog) cannot become sufficiently small, and also is susceptible to change depending on the storage state after preparation. Particularly, when stored under high temperature and high humidity, the minimum density becomes remarkably higher with lapse of time, resulting in impairment of the quality of white ground of the image to pose one problem in practically applying this kind of emulsion.
In the prior art, various antifoggants or development inhibitors have been known for the purpose of improving such undesirable phenomenon. For example, benzotriazoles described in U.S. Pat. No. 2,497,917, heterocyclic thione compounds described in Japanese Patent Publication No. 12709/1970, tetrazole type compounds described in U.S. Pat. No. 3,352,672 have been used as the inhibitors. However, even when these compounds may be used, inhibition against fog and negative image generated with lapse of time is insufficient to give no sufficient white ground, and also use of a large amount of inhibitor as suggested in the above patents is not desirable because development is remarkably inhibited to lower the image density.
Further, in the recent field of photography, it has been well known to shorten and expedite the processing time by performing development and the post-processing steps (desilverization, fixing, water washing, etc.) subsequent thereto at high temperature. When development is performed at a high temperature, for example, a temperature of 30.degree. C. or higher, the fog generated by storage with lapse of time is further amplified.